The most common stoppage of the drain line from the coil to the sewer is the trap. Heavy particles will settle to the bottom and eventually will clog. Just simply clean it out occasionally. Run a wire, flush with water, or blow air through it to clear the clog.

TIP
One of the very best ways to clear the drain line and the sewer line it dumps into is to pour scalding hot water into it. Use a liberal amount.

· Look for the presence of two traps inline. An air trap will sooner or later happen and it will be as though the line is clogged. If you find a sag in the tubing that makes the equivalent of a trap plus a real trap or any multiple combination of either of these in the same line it is a problem. Make sure the line drains consistently downward and only has one trap.
· Anytime the drain is attached at a point on the system where it is on the suction side of the air flow, it must have a trap or air/gasses may come up into the system. If this is gas this could become very dangerous. This condition of suction can also cause the water to back-up in the line and cause an overflow of the pan. Trap it if it doesn't have one.
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· If the evaporator iced and is thawing, it may run water from nearly any place. The system may have a dirty filter, restriction of air flow, be low on refrigerant, or have numerous causes for the icing. Refer to the menu and read the Refrigerants and Charging section where it is explained in great detail.

Sewer drain clogs may cause the condensation water from the air conditioner to back up and overflow. Use the hot water tip above to clear it.

If the drain pan is rusted out, it is a serious problem and you will have to replace the cooling coil and pan most likely. There are no repairs for a rusted pan.